A typical example of these drivers is disclosed in Japanese Patent Publication No. 3548497. The driver for an IGBT (Insulated-Gate Bipolar Transistor) as an example of semiconductor switching elements includes a diode and a resistive element comprised of a pair of series-connected resistors, and the diode and the resistive element are connected between the collector and emitter of the IGBT. Specifically, the anode of the diode is connected to one end of the resistive member, and the cathode is connected to the collector of the IGBT. The connection point between the one end of the resistive element and the anode of the diode is connected to the gate of the IGBT via a resistor. To the gate of the IGBT, a power supply source is connected for charging the gate.
The driver configured set forth above is adapted to measure an overcurrent flowing through the IGBT as a function of the variations in the potential at the connection point between the series-connected resistors of the resistive element. Next, how the driver measures an overcurrent will be described hereinafter.
When the IGBT is turned on, the power supply source charges the gate of the IGBT, resulting in an increase in the voltage at the gate. Thereafter, the IGBT is turned from ON to OFF.
If no overcurrent flows between the collector and emitter of the IGBT, the collector-emitter voltage becomes lower than the potential at the anode of the diode, so that the electrical charge stored on the gate of the IGBT are transferred from the gate to the collector via the diode. This discharge of the gate results in potential drop at the connection point between the diode and the resistive member. The potential drop causes the potential at the connection point between the series-connected resistors of the resistive member to drop down to a level close to the potential at the emitter of the IGBT.
In contrast, if an overcurrent flows between the collector and emitter of the IGBT, the collector-emitter voltage is kept at a high level despite the turn-on of the IGBT, so that the electrical charge stored on the gate of the IGBT remain thereon without being discharged from the gate to the collector via the diode. For this reason, the voltage at the gate of the IGBT is divided between the resistances of the series-connected resistors of the resistive member and the resistance of the resistor connected to the gate of the IGBT; the voltage division defines the potential at the connection point between the series-connected resistors. The potential at the connection point between the series-connected resistors becomes higher than the level close to the potential at the emitter of the IGBT.
Thus, the driver is adapted to determine whether an overcurrent flows through the collector-emitter of the IGBT as a function of the variations in the potential at the connection point between the series-connected resistors of the resistive member.